Transistor voltage regulator



Feb. 20, 1962 D. B. DOAN TRANSISTOR VOLTAGE REGULATOR Filed Feb. 19,1960 POSITIVE 9 TEMPERATURE 1 COEFFICIENT INPUT VOLTAGE l NEGATIVE I 3 BTEMPERATURE l COEFFICIENT l 4 J L i .4

INVENTOR David 5 00am ATTORNEYS 3,022,457 TRANSESTOR VOLTAGE REGULATORDavid B. Doan, Austin, Tex., assignor to Texas Instruments Incorporated,Dallas, Tex., a corporation of Delaware Filed Feb. 19, 1960, Ser. No.9,853 4 Claims. (Cl. 32322) This invention relates to transistor voltageregulators, and more particularly to transistor voltage regulatorsutilizing a novel arrangement of zener diodes interconnected to obtainvoltage stabilization over a wide temperature range.

One type of transistor voltage regulator, to which the principles of theinvention apply, is known as an emitterfollower transistor voltageregulator. In this type of regulator, the output or emitter voltage willremain nearly constant, provided the base voltage of the transistorremains nearly constant because the output or emitter voltage closelyfollows the base voltage.

In prior emitter-follower voltage regulators, zener diodes, which havethe characteristics of good voltage regulation and low-currentrequirement, have been utilized in the base of the transistor tomaintain the base voltage at a constant or stable reference voltage. Theuse of a single regulating zener diode in the base or" a transistorenables the base voltage to be maintained constant or stable providedthe temperature of the zener diode does not vary or change underoperating conditions. The voltage drop across the diode will vary,however, with changes in temperature because the regulating diodeexhibits a positive temperature coefficient of zener voltage. It isevident then, that under conditions of varying temperature, the basevoltage of the transistor will not stay fixed at a constant or stablereference voltage. To offset this difiiculty, it has been suggested toplace compensating zener diodes, exhibiting a negative temperaturecoefiicient of zener voltage, in series with the zener diode, exhibitinga positive temperature coellicient of zener voltage; the variationsproduced due to varyiir temperature will substantially cancel out over aWide range of temperatures. The use or" such compensating zener diodesbrings about the desired result of maintaining the base voltage of thetransistor constant or stable over a wide range of temperatures.

The use of these temperature-compensating zener diodes, however,although solving one difiiculty, gives rise to disadvantages anddrawbacks. These temperaturecompensating zener diodes have thecharacteristics of poor voltage regulation and high-current requirement;Because of the latter characteristic, the current supplied to thesediodes from the input voltage of the transistor regulator is notsuflicient to operate these diodes. Thus, the circuitry becomesinoperative as a regulator.

Another disadvantage of prior regulators is the relatively high internalcurrent consumption. When the input voltage of the regulator is variedthrough the minimumto-maximum regulating range, there will be a largeincrease in current. drawn from the power source.

A further disadvantage of prior regulators is the relatively highvoltage drop across the regulator "at minimum input voltage.

It is, therefore, the principal aim of this invention to provide asimple and inexpensive way to overcome the difficulties anddisadvantages arising from the use of temperature-compensating zenerdiodes having high-current requirement. This is accomplished by thepresent invention by providing a current-path means consisting of aresistor interconnecting the emitter or output terminal and the base ofthe transistor. Thus, the high current reates Patent- 3,h22,457 PatentedFeb. 20, 1962 quired to operate the temperature-compensatiug zenerdiodes is supplied from the output side of the regulator. Although thesezener diodes have poor voltage regulation, the voltage drop across thesediodes is not appreciably susceptible to change when the high currentflowing through them varies slightly with changes in input voltage. Withthe resistor providing a high-current path from the output side of theregulator, changes in input voltage will not cause appreciable currentchanges in the output side. Accordingly, the unique arrangement of thepresent invention provides a desirable voltage regulator stable over awide temperature range of operation.

It is, therefore, the main object ofthis invention to provide atransistor voltage regulator stable over a wide temperature range ofoperation which features novel circuit means including a plurality ofzener diodes interconnected in a unique manner to stabilize the voltageregulator over a wide temperature range.

Another object of this invention is to provide an improved voltageregulator, which can be easily and inexpensively made stable over a wideoperating temperature range.

A further object of this invention is to provide an improved voltageregulator having a relatively low internal current consumption ascompared to prior regulators.

A still further object of this invention is to provide an improvedvoltage regulator having a relatively lower voltage drop across theregulator at minimum input voltage than previously known regulators.

Other objects and advantages of the invention will become apparent fromthe following detailed description of a preferred embodiment of theinvention when taken with the drawing which shows in the sole FIGURE aschematic diagram of the preferred embodiment of the invention.leferring now to the sole figure, the best mode for carrying out theinvention is as follows. There is shown a transistor voltage regulatingcircuit known as 'an emitterfollower voltage regulating circuit. Theregulating circuit comprises an NPN type silicon transistor 10 having anemitter H, a base 12 and a collector 13. A shunt resistance Q isconnected between the collector 13 and base 12 of the transistor 10. Theregulating circuit has four zener diodes l, 2, 3 and 4 connected inseries with the base 12 of the transistor ll}; Zener diode 1, whosecathode is connected to the base 12 of the transistor Ill, has thecharacteristics of low-current requirement and good voltageregulation-its zener voltage does not change appreciably with change incurrent. It will, however, eX- hibit a positive temperature coefficientof zener voltage. Another zener diode means, comprising diodes 2, 3 and4-, is selected to have a negative temperature coefficient of zenervoltage. Since diodes with negative temperature coefficients of zenervoltage come only in small voltages, this diode means is shown asmultiple diodes. The tem perature-compensating zener diodes 2, 3 and 4,however, exhibit the characteristic of high-current requirement.

To supply these diodes with the required high current, a current-pathmeans is provided for the three temperature-compensating diodes 2, 3 and4, and consists of a resistor 14, interconnecting the emitter 11 and thecommon junction between the anode of diode 1 and the cathode of diode Zin the base circuit of transistor 10. This current path supplies thehigh current required to operate the temperature-compensating diodes 2,3 and 4 from the output side of the regulating circuit. Thus, changes ininput voltage will not cause appreciable current changes in the outputside. i

The current flowing through the temperature-compensating diodes 2, 3 and4 has a range, by way of example, of about 4 to 5 milliamperes. Thecurrent flowing through regulating diode i has a range, by way ofexample, of about 0.1 to l milliarnpere. A source 5 of input voltage isconnected across the input terminals 6 and '7. An impedance load 13,shown as a r sistor, is connected across the output terminals 16 and 17.it is appreciated that the impedance load 18 is not restricted to aresistor, but may be an inductor or other suitable circuit element. Inone test, the input voltage from source 5 was varied from 25 to 33 voltsand the output voltage across the load impedance 1S varied from to 21volts. The regulator operates over a wide temperature range, such as, byway of example, 20 C. to 83 C.

The operation of the apparatus will now be described. With the inputvoltage at volts, diode 1 draws 0.1 milliampere from the input voltage 5through resistor 9. Since this current is not sufiicient to operatediodes 2, 3 and 4, these diodes draw 4 milliamperes through resistor 14from the output or regulated side of the regulator. The output voltageacross the resistive load 13 is 20 volts. As the input voltage variesfrom 25 volts to 33 volts, there is a current change of approximately 1milliampere through the diodes 1, 2, 3 and 4. The current throughregulating zener diode 1 changes from 0.1 to l milliampere, and thecurrent through the compensating zener diodes 2, 3 and 4- changes from 4to 5 milliamperes. Although the current through diode It changesconsiderably, the voltage drop across regulating diode 1 does not changeappreciably because the diode 1 has the characteristic of good voltageregulation. Moreover, the voltage drop across the compensating diodes 2,3 and d, which have the characteristic of poor voltage regulation, doesnot change appreciably either because the current change through thesediodes is from 4 to 5 milliarnperes, which is too small a change tocause any appreciable change in voltage across them.

Since diode 1 draws its small current required from the input voltagethrough resistor 9, and diodes 2, 3 and 4 draw their large amount ofcurrent required through resistor 14 from the regulated side of theregulator, it is evident that there will be little total change incurrent through all of the diodes when the input voltage varies from 25to 33 volts. Now, as the temperature varies, the zener voltage of diode1 varies proportionally and oppositely to the zener voltage of diodes 2,3 and 4. When this event occurs there is very little over-all change inzener voltage at the base 12 of transistor 1%. The base voltage and theemitter or output voltage of the transmitter iii, therefore, aremaintained at a desired constant or stable voltage, since the emittervoltage follows the base voltage in the emitter-follower voltageregulator. With the input voltage changing from 25 to 33 volts, theoutput voltage across the load impedance 18 changes slightly from 20 to21 volts. In the constructed embodiment, the regulator operated with aminimum of five volt drop at a minimum input voltage of 25 volts, andthe output voltage remained nearly constant. However, with other typesof transistors and with other diodes, the minimum operating voltage andother circuit characteristics may vary.

Thus, in operation it can be seen that there is very little internalcurrent consumption when the input voltage is varied through theminimum-to-maximum regulating range. Although prior regulators operatedwith a minimum voltage drop of at least 8 to 10 volts across theregulator per 25 volt input, the regulator of the present invention isdesigned to operate with a minimum of 5 volt drop at a minimum inputvoltage of 25 volts.

The total regulator is relatively low cost and requires very fewcomponents for its operation-one transistor, four zener diodes and twoinexpensive resistors.

This voltage regulator has good regulation over a wide range oftemperature and input voltage. The regulator has the ability to regulatewithin plus or minus 1.3% with an input variation of plus or minus 10%through a temperature range of minus 20 C. to plus 83 C.

This invention is especially useful for silicon transistors, although itcan be adapted for use with other semiconductor transistors suchgermanium. However, in the case of germanium transistor regulators. itis known that they can be made more easily in other ways because of thelower collector resistance.

It is appreciated that the diode i, could be picked to have a negativetemperature coer'ficient of zener voltage. In this case, the other diodemeans would be selected to have an opposite temperature coelficient.

Moreover, although the use of three zener diodes in series has beenfound expedient because or". the inherent limitations in commerciallyavailable diodes, if more desirable diodes were available, one mightreplace the three shown (diodes 2, 3 and 4 in the figure).

The diode 1 has been specified to have a positive temperaturecoefficient of zener voltage, whereas the diodes 2, 3 and 4- have beenspecified to have a negative temperature coefiicient of zener voltage. The selection of the specific diodes used will be a matter of choice toone skilled in this art. By way of example, an operating circuit may beconstructed by employing a commercially available diode identified inthe trade as type 1N753 tor diode 1 on the drawing; this diode has apositive ternpcrature coefiicient of +.022% per degree centigrade. Thediodes numbered 2, 3 and 4 on the drawing may each be the commercialtype 1N75l which has a negative temperature coefficient of .008% perdegree centigrade. The voltage regulation characteristics of each ofthese diodes may be had by reference to standard textbooks or to themanufacturers specification sheet on the diode selected.

Although the present invention has been shown and described in terms ofa specific preferred embodiment, changes and modifications which do notdepart from the inventive concepts taught herein will suggest themselvesto those skilled in the art. Such changes and modifications are deemedto fall within the scope and contemplations of the invention.

What is claimed is:

1. A transistor voltage regulator of the emitter-fob lower typecomprising a transistor having an emitter, a base, and a collector,first zener diode means having one type of temperature coefilcient ofzener voltage connected to said base, second zener diode means having anopposite type of temperature coefficient of zener voltage connected inseries with said first diode means, and means establishing a currentpath interconnecting said emitter and the common junction of said firstand second zener diode means.

2. In a transistor voltage regulator of the emitter-follower typecomprising a transistor having an emitter, a base and a collector, firstzener diode means having one type of temperature coefficient of zenervoltage connected to said base, means establishing a current pathconnecting said collector and said base to supply a small current tosaid first zener diode means, second zener diode means having anopposite type of temperature coefiicient of zener voltage connected inseries with said first zener diode means, and current-path meansinterconnecting said emitter and the common junction of said first andsecond zener diode means.

3. A transistor voltage regulator of the emitter-follower typecomprising a silicon transistor having an emitter, a base, and acollector, first zener diode means having a positive temperaturecoefiicient of zener voltage connected to said base, second zener diodemeans having a negative temperature coefficient of zener voltageconnected in series with said first diode means, and means estab lishinga current path interconnecting said emitter and the common junction ofsaid first and second zener diode means.

4. In a transistor voltage regulator of the emitter follower typecomprising a silicon transistor having an emitter, a base and acollector, first zener diode means emitter and the common junction ofsaid first and second zener diode means.

References Cited in the file of this patent Designing TransistorCircuits-DC. Regulators, R. B. Hurley, Electronic Equipment, April 1957,pp. 20-23.

